The present invention relates to variable gain mixers, and more precisely, to an exponentially variable gain mixer.
A mixer is a circuit that converts the frequency of a signal, and is used in modern transceivers in telecommunication networks. Because of the attenuation to which a transmitted signal may undergo along the path from a transmitter to a receiver, the gain of the receiver needs to be increased to produce a signal having a constant magnitude. Similarly, the transmitter needs to be able to reduce the power of the transmitted signal to make the receiver correctly receive other signals having a relatively small magnitude.
The two functions of modulation and amplification with variable gain may be carried out by two distinct circuits, commonly called a mixer and a variable gain amplifier, respectively, or may be carried out by a single circuit commonly called a variable gain mixer (VGM). The latter approach is often preferred because of reduced current dissipation, enhanced noise immunity and reduced nonlinear distortion. Moreover, input and output signals in a variable gain mixer have different frequencies, and this favors the isolation between the input and the output. This characteristic is particularly important if the VGM needs to significantly attenuate the output signal.
The gain G of the VGM is generally a function of a gain control voltage VG, that is, G=G(VG). Very often an exponential variation law is required in transceiver design in order to have a linear variation of the gain expressed in dB, GdB=20xc2x7Log(G). To obtain this type of function, the control voltage VG needs to be generated by a circuit with an appropriate transfer characteristic.
One embodiment of the VGM is described in U.S. Pat. No. 5,933,771 to Tiller et al., and is depicted in FIG. 1 herein. The differential current signal (isig, xe2x88x92isig) is the input of a modified version of a so-called Gilbert Cell, formed by the transistors Q1, Q2, Q3 and Q4. The differential current modulates the frequency based upon application of a differential voltage signal (Vlo+, Vloxe2x88x92) generated by an oscillating circuit.
Assuming that a differential voltage signal has a square waveform, then only two of the four transistors Q1-Q4 are on during every half-period. The final result is a multiplication of the input current isig by +1 during a half-period, and by xe2x88x921 during the successive half-period.
The current regulating transistors Q5 and Q6 are identical, and these are driven by the gain control voltage VG. Their function is to shunt part of the current of the input signal (isig, xe2x88x92isig) towards the positive supply Vpos, thus causing an attenuation as a function of the voltage VG. When the voltage VG assumes its minimum value, the transistors Q5 and Q6 are turned off and all the signal current isig is modulated at the output (iout+, ioutxe2x88x92) . On the contrary, when the voltage VG assumes its maximum value, all the signal current flows in transistors Q5 and Q6, and thus there is no output current. In general, the attenuation, that is, the ratio between the output current signal iout and the input current signal isig, may be expressed by the following equation:                                           i            out                                i            sig                          =                  1                      1            +                          e                                                VG                  -                  Vlo                                                  V                  T                                                                                        (        1        )            
wherein VT is the thermal voltage. As it may be noted, a drawback of this approach is the fact that the attenuation depends, besides from the voltage VG, but also from the amplitude of the differential signal Vlo, which, in general, is independent from the voltage VG.
In an attempt to solve this problem, the circuit depicted in FIG. 2 has been proposed in the above cited prior art reference. An oscillating circuit 3, comprising a pair of oscillators and a relative bias circuit BIAS_CIRCUIT, generates an alternating differential signal that is input to the variable gain mixer VGM as a differential modulation signal (Vlo+, Vloxe2x88x92). The peak detector PEAK_DETECTOR included in the correction circuit 2 receives the alternating differential signal and generates the respective peak voltage Vlo.
An externally generated voltage for commanding the variation of the gain VG2 is added to the peak voltage Vlo and to the bias voltage Vb of the oscillators of the circuit 3. In this way, the voltage VGxe2x88x92Vlo, which (see equation (1)) determines the attenuation of the output signal iout with respect to the input signal isig, does not depend anymore from the peak voltage Vlo:
VGxe2x88x92Vlo=(VG2+Vb+Vlo)xe2x88x92Vlo=VG2+Vbxe2x80x83xe2x80x83(2)
The drawback of the circuit of FIG. 2 is that it includes a peak detector that introduces delays and adds noise, and could require non-integrable elements.
An object of the present invention is to provide an exponentially variable gain mixer that has a very straightforward structure, is not affected by the above mentioned noise and delay problems, and does not require the use of a peak detector for its functioning as in the prior art circuits.
This and other objects, advantages and features in accordance with the present invention are provided by an exponentially variable gain mixer comprising a variable gain mixer receiving an input differential signal to be amplified, and producing an amplified differential output signal that is a function of a differential modulation signal and a control voltage. The mixer further comprises an oscillating circuit for generating an alternating differential signal. A correction circuit is input with an external gain variation command, and with the alternating differential signal for producing the differential modulation signal and the control voltage.
Unlike comparable known circuits, the modulator of the invention does not require a peak detector because the correction circuit comprises a first amplifier input with the external gain variation command that generates the control voltage and a bias voltage as a differential output signal. The modulator includes a second differential amplifier, referenced to the bias voltage, and is input with the alternating differential signal and outputs the differential modulation signal.
Preferably, the gain of the first amplifier input with the external command varies proportionally with the operating temperature, thus making transfer characteristics of the mixer substantially independent from the temperature.
According to another embodiment, the modulator of the invention is provided with a compensation circuit for coupling the first amplifier to the second amplifier such that the attenuation may vary exponentially with a very high precision.